The present invention relates to a graphite powder having a novel microstructure, which is suitable for use as a carbon material for negative electrodes of lithium ion secondary batteries. More specifically, the present invention pertains to a graphite powder capable of producing negative electrodes for lithium ion secondary batteries having a high discharge capacity and an improved charge/discharge coulombic efficiency and to a process for the preparation of such a graphite powder. It is also concerned with a material for negative electrodes of lithium ion secondary batteries and a lithium ion secondary battery having a negative electrode formed from such a material.
Lithium secondary or storage batteries are a class of nonaqueous secondary batteries using lithium as an active material for a negative electrode, an oxide or chalcogenide (e.g., sulfide or selenide) of a transition metal as an active material for a positive electrode, and a solution of an inorganic or organic lithium salt dissolved in an aprotic organic solvent as an electrolyte.
Since lithium is a base metal having a very low electric potential, use of lithium as a negative electrode in a battery provides the battery with the ability to readily obtain a high potential therefrom. For this reason, lithium secondary batteries have recently attracted increasing attention as promising secondary batteries having a high electromotive force and a high energy density, and they are expected to find applications as distributed batteries or portable batteries in a wide variety of fields including electronic equipment, electric equipment, electric vehicles, and electric power storage. Lithium secondary batteries have already been put into practical use as compact batteries.
Initially, lithium metal in the form of foil was used to form a negative electrode by itself in lithium secondary batteries. In such cases, the discharging and charging reactions proceed with dissolution (ionization) and deposition of lithium. In the reaction of Li+xe2x86x92Li during charging cycles, however, the metallic lithium tends to deposit as acicular crystals on the negative electrode, and repeated discharging and charging cycles cause the formation of lithium dendrite (tree-like branching crystals) on that electrode. As the lithium dendrite grows, it may break through the separator of the battery, leading to an internal short circuit by direct contact of the dendrite with the positive electrode. Therefore, these batteries have a fatal drawback of a very short cycle life in a repeated discharging and charging cycle test.
In order to eliminate this problem in lithium secondary batteries having a negative electrode of lithium metal, it was proposed to use carbon materials (e.g., naturally-occurring graphite, artificial graphite, petroleum coke, carbonized resins, carbon fibers, pyrolyzed carbon, carbon black, and the like), which are capable of reversibly receiving and releasing lithium ions, to form a negative electrode of these batteries [see, e.g., Published Unexamined Japanese Patent Application No. 57-208079(1982)]. In such batteries, the material for the negative electrode may be comprised substantially entirely of the carbon material. Such a negative electrode can be manufactured by attaching the carbon material in powder form to a metal base serving as a current collector, normally with the aid of a suitable binder.
The electrode reactions of a lithium secondary battery having a negative electrode made of a carbon material have not been elucidated completely but may be considered to be as follows. While the battery is charged, electrons are delivered to the carbon material of the negative electrode, thereby causing the carbon material to be negatively charged. The electrolyte contains lithium ions, which are attracted toward the negatively charged carbon material of the negative electrode and are received therein by an electrochemical intercalation reaction. Conversely, during a discharging cycle, the lithium ions contained in the carbon material are removed (deintercalated) from the negative electrode to release them into the electrolyte solution. Thus, charge and discharge occur by receipt of lithium ions into the negative electrode material and release of them from the material. In view of this mechanism, this type of battery is generally called a lithium xe2x80x9cionxe2x80x9d secondary battery. Lithium ion secondary batteries do not involve the deposition of metallic lithium on the negative electrode during electrode reactions, thereby avoiding the problem of deposition of lithium dendrite, which deteriorates the negative electrode significantly. Lithium secondary batteries which are commercially used at present are mostly of this type, i.e., of the type having a negative electrode of a carbon material.
The theoretical capacity of a lithium secondary battery having a negative electrode of lithium metal is very high, i.e., on the order of 3800 mAh/g. In the case of a lithium ion secondary battery having a negative electrode of a carbon material which receives lithium ions therein, its theoretical capacity is limited to 372 mAh/g even when the negative electrode is comprised of a lithium-graphite intercalation compound (C6Li), which is a graphite (a highly crystalline carbon material) having lithium ions densely and regularly incorporated in the interstices between the layer crystal lattices of graphite.
In practice, however, the carbon material used as a negative electrode has surface active sites which interfere with entry of lithium ions and dead regions incapable of receiving lithium ions. Therefore, even if a highly crystalline graphite is used to form a negative electrode of a lithium ion secondary battery, it is extremely difficult to achieve a capacity of 372 mAh/g, the theoretical capacity of C6Li.
In addition, when a highly crystalline carbon material or graphite is used to form a negative electrode, the surface of the electrode has a higher reactivity than the inside thereof since the crystal structure is interrupted on the surface, and a passivated film tends to deposit on the more reactive surface as a component of the electrolyte is slightly decomposed by the action of a high charge voltage. The quantity of electricity consumed for the decomposition is lost wastefully, thereby decreasing the charge/discharge coulombic efficiency (ampere-hour efficiency) of the electrode, i.e., the ratio of discharged to charged quantity of electricity, which is an indication of the performance of a secondary battery, calculated by the equation [(discharge capacity)/(charge capacity)xc3x97100 (%)]. The use of such a material requires that a battery be designed using an extra amount of material for the positive electrode to allow for the decrease in charge/discharge coulombic efficiency. This is disadvantageous for applications such as compact batteries which have a given shape defined by specifications.
In order to increase the discharge capacity of a carbon negative electrode for lithium ion secondary batteries to as close to the above-described theoretical capacity as possible, various methods have been proposed for the production of a carbon material for the negative electrode.
For example, Published Unexamined Japanese Patent Applications Nos. 4-115458 (1992), 5-234584 (1993), and 5-307958 (1993) disclose the use of a carbonized product of mesophase microbeads which are formed in the course of carbonization of pitch. The mesophase microbeads are liquid crystalline spherical particles exhibiting optical anisotropy and are formed by subjecting pitch to heat treatment for a few hours or more at about 400-550xc2x0 C. When the heat treatment is further continued, the microbeads are grown and finally united with each other to form a mass, called a bulk mesophase, which exhibits optical anisotropy as a whole. The bulk mesophase may be used as a material for carbonization. However, the carbonized products of these materials, when used as a negative electrode, do not have a sufficiently high discharge capacity.
Published Unexamined Japanese Patent Application No. 7-282812 (1995) states that the capacity of a lithium ion secondary battery having a negative electrode made of graphitized carbon fibers can be enhanced by increasing the regularity of the aligned layer structure of graphite crystal lattices in the negative electrode. It is pointed out therein that pulverization of the carbon fibers introduces undesirable structural defects which disturb the regularity of the aligned layer structure of the graphitized carbon fibers. The increased regularity of the aligned layer structure of graphite crystal lattices provides the negative electrode with a discharge capacity of at most 316 mAh/g, but cannot produce a carbon material for a negative electrode having a sufficiently high capacity, e.g., one exceeding 320 mAh/g.
Published Unexamined Japanese Patent Application No. 6-187972(1994) discloses a carbon material produced by calcination at a high temperature for carbonization of a particular resin formed by reacting an aromatic reactant with a crosslinking agent in the presence of an acid. The resulting carbon material has a microstructure comprising crystalline phases formed by crystallization of the aromatic reactant, which are intermingled with amorphous phases derived from the crosslinking agent. These two types of phases have different indices of thermal expansion and shrinkage, which causes the carbon material to have numerous internal structural defects in the form of voids. It is described in this application that the carbon material gives a high capacity when used to form a negative electrode of a lithium ion secondary battery, since absorption of lithium metal by the voids also occurs, in addition to interlayer absorption of lithium ions due to the above-described intercalation (formation of C6Li). This carbon material is disadvantageous from a cost standpoint since its material costs are high due to the use of a special resin for carbonization. Furthermore, it cannot provide the negative electrode with improved charge/discharge coulombic efficiency.
It is an object of the present invention to fabricate a lithium ion secondary battery having a high discharge capacity and preferably an improved charge/discharge coulombic efficiency, using a carbon material which can be produced from a conventional inexpensive raw material rather than from a special resin.
A more specific object of the present invention is to provide a graphite powder capable of receiving an increased amount of lithium ions therein, which, when used as a material for the negative electrode of a lithium ion secondary battery, can achieve a high discharge capacity on the order of at least 315 mAh/g, preferably at least 320 mAh/g, more preferably at least 330 mAh/g, and under some conditions, 350 mAh/g or more, in a stable manner.
The inventors of the present invention systematically investigated the relationship between the microscopic structure of a graphite powder and its charge/discharge characteristics and analyzed it in various ways using theoretical calculations. As a result, it has been found that the surface of a graphite powder has xe2x80x9cclosed-end structuresxe2x80x9d formed by closing pairs of c-plane layers of the graphite layer lattices at their ends during heat treatment for graphitization. As schematically shown in FIG. 1, the closed-end structures on the surface of the graphite powder (surface closed-end structures) are in a laminar form constituted by several pairs of c-plane layers which are closed at their ends around adjoining layers. Between two adjacent laminar closed-end structures, there remains an xe2x80x9cintersticexe2x80x9d which is open at the end of the c-plane layers.
It has also been found that the density of the open interstices (which corresponds to the density of the laminar closed-end structures) significantly influences the discharge capacity of a lithium ion secondary battery having a negative electrode formed of the graphite powder. Namely, the discharge capacity can be improved by increasing the density of the open interstices, thereby making it possible to achieve the above-described desired values for discharge capacities. In addition, it has been noted that the density of the open interstices can be increased by the conditions for pulverization of a carbon material prior to graphitization or by heat treatment after graphitization.
The present invention provides a graphite powder characterized by having surface closed-end structures in which the graphite c-plane layers have closed-ends on the surface of the graphite powder formed by linking the ends of pairs of c-plane layers while leaving interstices which are open on the surface of the graphite, the number of the open interstices being at least 100 and at most 1500 per micrometer measured in a c-axis direction of the graphite. Preferably, the graphite powder has a specific surface area of 1.0 m2/g or less.
The graphite powder according to the present invention can by prepared by a process comprising subjecting a carbon material, which has been carbonized and pulverized at a high speed before and/or after the carbonization, to heat treatment at a temperature of 2500xc2x0 C. or higher for graphitization.
Alternatively, the graphite powder can also be prepared by a process comprising subjecting a graphite powder to heat treatment under conditions that can remove the surface of the graphite and subsequently to additional heat treatment at a temperature of 800xc2x0 C. or higher in an inert gas. The graphite powder used in this process as a starting material (to be subjected to the heat treatments) may be a powder of either artificial or natural graphite. The artificial graphite powder may be prepared by subjecting a carbon material, which has been carbonized, to heat treatment at a temperature of 2500xc2x0 C. or higher for graphitization and performing pulverization before or after the carbonization or after the graphitization. The natural graphite powder may be prepared merely by pulverizing naturally-occurring graphite. This second process can prepare a graphite powder according to the present invention which has a significantly increased density of open interstices, i.e., a very large number of open interstices per micrometer. The heat treatment under conditions that can remove the surface of the graphite is preferably oxidative heat treatment.
The present invention also provides a material for negative electrodes of lithium ion secondary batteries, comprising predominantly the graphite powder according to the present invention, and a lithium ion secondary battery having a negative electrode produced using such material.